I had unsolved ffmpeg UDP stream smearing problem for more than two years, with zm-1.30.0 installed and working 800 days without a single touch!
That was not so huge problem, as I set source to "remote", and there the video is clean.
As recent ZM from GIT supports h264 passthrough recording, thus saving 85% data storage, I decided to replace it.
All for me vital functions works excellent, so I do not see any reason to not to use it as my main surveillance system for my home house.
So to record passthrough, ffmpeg UDP is a must for me.
After endless googling, I finally found https://github.com/ZoneMinder/zoneminder/issues/
The key part is monitor settings, tab 2 (source) Options. By adding here
the smearing and frame drops completely disappear. This saves me 100€ as I was almost forced to buy a new camera, proved to work OK.buffer_size=10000000